(a) Field of the Invention
Broadly speaking, this invention relates to telecommunications. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for providing engineering orderwire channels over telecommunications systems employing time-division multiplexing.
(B) Discussion of the Prior Art
As is well known, it is customary to provide at least one engineering orderwire for each multiplex telecommunications system that is placed into service. The orderwire is used by maintenance personnel located at intermediate repeater stations along the route and at both terminal locations to coordinate routine maintenance and trouble-shooting on the associated carrier system.
Ideally, the orderwire is completely divorced from the carrier system and follows a different physical route. Thus, in the event of a total system failure, the orderwire will remain functional, which greatly facilitates restoration of the failed carrier system.
Unfortunately, the provision of an independent, separately-routed orderwire is expensive to implement and in many situation, such as those where the carrier system is to be used in a tactical military environment, is impossible to achieve. Accordingly, the orderwire must somehow be routed along the same path that the carrier system follows. Of course, the orderwire may itself comprise one of the multiplexed channels in the carrier system but, if the multiplexing equipment should fail, then the orderwire will also fail, making system restoration most difficult.
On the other hand, if the carrier system is an analog system operating over metallic conductors or coaxial cable, the orderwire may be transmitted at baseband, completely bypassing the multiplexing equipment which considerably reduces the chance of simultaneous failure. Thus, unless the cable itself fails, the orderwire will always be available to coordinate restoration of the basic system.
A problem arises, however, when the carrier system is digital rather than analog. While, in theory, the orderwire can still be transmitted as a baseband signal, the large number of digital repeaters and their transmission characteristics makes this most impractical to implement. Further, if the digital signal is transmitted over an optical fiber, rather than a metallic conductor, practical difficulties arise in modulating the optical source in both analog (baseband) and digital (pulse data stream) modes.